Heart failure (HF), which is the end-stage form of multiple cardiovascular diseases, remains a leading source of morbidity and mortality in the United States and worldwide. Progression to HF often results from adverse structural and functional remodeling of the heart following myocardial infarction (MI). This is characterized by the progressive loss of viable cardiomyocytes due to ischemic injury and replacement with a fibrotic scar that is unable to support the contractile needs of the heart, leading to functional decompensation, pathological hypertrophy, and failure. Historically, the adult mammalian heart was thought to lack the capacity to generate new cardiomyocytes following postnatal growth or after injury, relying exclusively on tissue fibrosis and scarring as the only means of healing. However, growing evidence over the past two decades has now demonstrated a modest degree of cardiac regenerative potential in adult mammals including humans, suggesting that regeneration of lost myocardial tissue is a potential therapeutic avenue for limiting HF progression. However, the endogenous regenerative capacity of the heart appears insufficient to resolve the massive injury that occurs post-MI. More importantly, the underlying molecular mechanisms of endogenous cardiac regeneration remain unresolved. The isolation and characterization of several types of cardiac-resident progenitor cells (CPCs) in the adult heart using antigenic markers (c-Kit, Sca-1 and the side population [SP] cell marker ABCG2) has led to considerable attention (and several clinical trials) on resident cardiac stem/progenitor cells a potential significant contributors to cardiomyocyte turnover and replacement in the adult heart. However, as-yet there is no consensus as to the extent that CPCs contribute new cardiomyocytes to the adult heart under physiological or pathological conditions. This is in part because the field has largely employed ex vivo isolation, expansion, and transplantation of CPCs, but no definitive genetic studies have been performed to determine the biological role of these cells in vivo, within their endogenous niches. This proposal seeks to address this issue and define the extent that two classes of CPCs - Sca-1+ cells and ABCG2+ SP cells - contribute new myocytes to the adult heart during normal physiological growth or post-MI. Previous studies have demonstrated that Sca1+ and ABCG2+ CPCs possess some degree of cardiomyogenic potential in vitro. Using transgenic mouse models generated in the Sponsor's lab, we will employ a genetic lineage tracing approach to quantitatively assess the contribution and functional significance of these CPCs to new cardiomyocyte formation in vivo, during 1) normal physiological growth and aging or 2) after MI injury. The Sponsor's lab has recently employed this genetic approach to define the contribution of c-Kit+ CPCs (van Berlo et. al. Nature. 2014), and found that c-Kit+ cells contribute primarily vasculature but a minimal degree of cardiomyocytes to the heart with aging and injury. Taken together with these data, our proposed studies will address a long-standing central question in cardiac regeneration: to what extent do resident cardiac progenitor cells (CPCs) contribute to the endogenous capacity of the adult heart to regenerate during aging or after injury? These studies will serve to both elucidate the molecular mechanisms underlying the intrinsic regenerative capacity of the adult heart, and provide insight as to potential therapeutic application of CPCs in restoring damaged myocardium post-MI, mitigating the progression to HF.